1. Field of the Invention
The present invention relates to a plasma treating apparatus for providing a plasma treatment, such as film forming or etching, to an object to be treated, such as a semiconductor wafer or the like.
2. Background Art
Generally, in order to produce semiconductor integrated circuits, various treatments including film forming, etching, oxidation, diffusion, modification and removal of naturally oxidized membranes are performed. In the case of performing these treatments in a vertical type, such as the so-called batch type heating apparatus, semiconductor wafers are transferred from a cassette capable of containing multiple sheets, for example 25 sheets, of semiconductor wafers onto a vertical type wafer boat, wherein the respective wafers are supported on the wafer boat in a multi-staged fashion. Depending on the wafer size, the wafer boat can contain 30 to 150 sheets of wafers. Thereafter, the wafer boat is loaded in a processing chamber from below, which chamber is capable of exhaustion, and the interior of the processing chamber is then kept airtight. Then, a predetermined heating treatment is performed while controlling various process conditions, such as flow rates of processing gases, processing pressure, processing temperature or the like.
Recently, needs for more highly integrated and refined semiconductor integrated circuits have been increased, and reduction of heat history in the production process for semiconductor integrated circuits is also desired in view of enhancement of the properties of circuit elements. Under such circumstances, various processing apparatuses utilizing plasma have been proposed because of possibility to perform an intended treatment, even without subjecting wafers to a significantly high temperature, in a vertical batch type processing apparatus (e.g., see Patent Documents 1, 2, 3, 4, 5).
For example, in a conventional plasma treating apparatus, for example, a pair of electrodes are provided respectively opposed to the center of a cylindrical processing chamber able to be vacuumed, on the outside of a side wall of the processing chamber. One of the electrodes is connected with a high frequency power source for generating plasma, and the other electrode is grounded, so as to generate plasma in the whole interior of the processing chamber by applying a high frequency voltage between both the electrodes. The semiconductors wafers are supported in a multi-staged fashion at an approximately central portion in the processing chamber, and a gas nozzle adapted to introduce a gas used for generating, for example plasma, is arranged on one side of the wafers. Thus, the wafers can be subjected to a plasma treatment by a heater provided surrounding the outer periphery of the processing chamber while being maintained to be heated at a predetermined temperature.
Now, a high frequency circuit including the aforementioned high frequency power source will be described. FIG. 8 is a diagram showing an equivalent circuit for a conventional high frequency circuit including a high frequency power source. In FIG. 8, reference numerals 2A, 2B denote a pair of plasma electrodes provided on the side of a processing chamber. Both of the plasma electrodes 2A, 2B are connected with a high frequency power source 6 of 13.56 MHz, for example, via wirings 4. By applying a high frequency between these plasma electrodes 2A, 2B, plasma P is generated between both the plasma electrodes 2A, 2B in a vacuumed state. In this case, the plasma P acts on the high frequency electrode 6 as a load and can be expressed equivalently as a series circuit consisting of a capacitor C, a coil L and a resistance R. Either one of the two plasma electrodes, for example, the lower plasma electrode 2B in the drawing, is grounded.
In the middle of the wirings 4, a high frequency matching circuit 8 is provided to perform matching of impedance in order to cancel a reflected wave from the load due to the plasma P, thereby to enhance efficiency of plasma generation. The high frequency matching circuit 8 comprises a first variable capacitor C1 connected in series with the plasma electrode 2A, a first coil L1, and a second variable capacitor C2 connected in parallel with the load of the plasma P on the side of the high frequency power source 6. It should be appreciated that the high frequency matching circuit 8 in the connected state described above is generally referred to as an inversed L type matching circuit.
In the high frequency matching circuit 8, by automatically adjusting the first and second variable capacitors C1, C2 to cancel the reflected wave on the side of the load of plasma P, the matching of impedance can be performed. As an alternative of the high frequency matching circuit 8, for example as shown in FIG. 9(A), it may be constructed with the first and second variable capacitors C1, C2 respectively connected in series with the load of plasma P, and a first variable coil L2 connected with the connection point between the two variable capacitors C1, C2 as well as connected in parallel with the load of plasma P via the other wiring 4. Otherwise, as shown in FIG. 9(B), the frequency matching circuit 8 may be constructed with a first and a second variable coils L2, L3 respectively connected in series with the load of plasma P, and a capacitor C3 connected with the connection point between the two variable coils L2, L3 as well as connected in parallel with the load of plasma P via the other wiring 4. It should be appreciated that the circuit construction as shown in FIG. 9 is generally referred to as a T type matching circuit.    Patent Document 1: TOKUKAIHEI No. 3-224222, KOHO    Patent Document 2: TOKUKAIHEI No. 5-251391, KOHO    Patent Document 3: TOKUKAI No. 2002-280378, KOHO    Patent Document 4: TOKUKAI No. 2001-44780, KOHO    Patent Document 1: TOKUKAI No. 2003-264100, KOHO
Because the plasma electrode 2B is grounded, a voltage Vab applied between both the plasma electrodes 2A, 2B in the high frequency matching circuit 8 described above is a sinusoidal wave as shown in FIG. 10.
In such a high frequency circuit, high production efficiency of highly powered plasma is required. However, in the high frequency circuit as described above, the plasma density to be obtained is not so high, thus the plasma production efficiency can not be enhanced sufficiently as desired.